Oxidation of trichloroethylene (TCE) and tetrachloroethylene (PCE) with various dosages of ozone or ozone plus hydrogen peroxide was studied in laboratory experiments. The results show that hydrogen peroxide accelerates the oxidation of TCE and PCE by ozone. At peroxideto-ozone dosage ratios of >0.7 (w/w), the process appears to be mass transfer limited. High levels of bicarbonate ion in the groundwater significantly decreased the efficiency of TCE and PCE removal by the ozone-peroxide system, suggesting that softening prior to oxidation may improve the process. Includes 19 references, tables, figures.